Considered a weapon of mass destruction, it is banned under the 1993 chemical weapons convention.
Here's what we know about it.
Nerve agents are the most toxic and fast-acting substances known in chemical warfare and, according to the US Centers for Disease Control and Prevention (CDC), VX is the most potent of all of them, including Sarin, which was developed in Germany in 1938 as a pesticide.
Nerve agents like VX are chemically similar to pesticides, although far stronger.
It was first developed in the UK in the 1950s as a chemical warfare agent. Experts say it has been replicated in the past by the US, Russia, Syria and Iraq.
It is an oily liquid, amber in color, odorless and tasteless.
The vapor form of VX is the deadliest -- and the quickest to kill. As a liquid, it could potentially be released into a water supply or used to poison someone's food.
Like all nerve agents, VX stops a vital enzyme from working -- which eventually leads to the body tiring, and no longer being able to breathe.
VX is not only the deadliest nerve agent, but also the most persistent in the environment -- it evaporates slowly, especially in cold conditions, making it both a long- and short-term threat.
What are the symptoms?
Depending how much of the agent a person was exposed to, symptoms will start occurring either immediately or up to 18 hours later.
Large doses can cause convulsions, loss of consciousness, paralysis and death, because of respiratory failure.
Smaller, non-fatal doses can cause a wide range of symptoms that include increased heart rate, blurred vision, nausea, diarrhea, drooling, pain and weakness.
Even just small doses can cause confusion and drowsiness.
There are antidotes for VX exposure available and they are most effective when administered immediately.
Who has it?
VX nerve agent was first used during the Iran-Iraq war in the 1980s. It's part of the same family of toxic substances as Sarin, which was used in the 1995 nerve gas attack on the Tokyo subway.
In the attack, members of the Aum Supreme Truth cult released Sarin nerve gas that killed 12 people and made more than 5,500 commuters sick.
VX is said to be relatively easy to produce in a reasonably sophisticated laboratory.
"Any country with a sophisticated chemical weapons effort can produce VX," said CNN's military analyst Rick Francona. "The formula has been around since the 1950s so its nothing new, it just has to be made, the political will and determination that we are going to build this weapon."
"Evidently the North Koreans have it and managed to weaponize it."
According to Francona, the US, British, Russians and Iraqis were all in possession of the nerve agent.
"(The US) is in the process of destroying our stocks," says Francona. "We were worried about VX getting into the hands of unstable characters."
Is it banned?
VX is prohibited under a number of international agreements, including the 1925 Geneva Protocol and the 1993 Chemical Weapons Convention.
The 1925 Geneva Protocol came about after the use of poisonous gas in World War I, which was later extended by the 1993 Convention to include a ban on their development, production, stockpiling, retention and transfer of chemical weapons.
"Because VX is so much more toxic than Sarin, it is considered a chemical weapon," says Corey Wallace, East Asia Security Policy Analyst at Freie University, Berlin. "There are no non-military uses for this agent."
Could North Korea use it more widely?
North Korea is one of the few countries that isn't a signatory to the Chemical Weapons Convention, which entered into force in 1997. But it is party to the Geneva Protocol, which prohibits the use of chemical weapons in warfare.
Pyongyang forcefully denies that it was behind the murder of Kim Jong Nam, and claims he died of "heart stroke." State media maintains that Malaysia's investigation has been politically influenced by South Korea.
But North Korean leader Kim Jong Un is renowned for his volatility. North Korea conducted two nuclear tests in 2016 -- it's most powerful ever in September
-- and in February claimed it had successfully launched a new medium long-range ballistic missile.
A string of missile tests, 340 reported executions and a slew of threats has the world worried about what exactly the erratic leader of the world's most reclusive nation is capable of. But if North Korea did produce the VX, experts say it's quite difficult to weaponize.
Any kind of long-distance missile transference would be ineffective, says Neil Wheate, chemical weapons health expert at the University of Sydney, because the agent would be lost in the blast. Likewise, he says, with a nuclear weapon.
The agent could be dropped from an aircraft, says Wheate, but has no capacity long-distance. "Not anything intercontinental," he adds.
However, if North Korea is behind this murder, experts say there's still cause for concern.
"On the back of the DPRK's recent advances in nuclear and missile capabilities, this sends a big signal to the international community about its military reach," says East Asia Security Policy Analyst, Corey Wallace.
If North Korea did use VX to murder Kim Jong Nam in Malaysia, he says it suggests a willingness to use weapons on foreign soil.
"This goes beyond the more traditional question of 'deterrence' against foreign enemies for which North Korea's nuclear weapons program is arguably being developed," he says.